I. Field of the Invention
The present disclosure generally relates to the field of semiconductors and semiconductor processing, and in particular, this disclosure provides, for example, non-aggregating metal oxide nanoparticles that may be used in some embodiments for the production of a high index immersion fluid for photoresists.
II. Description of Related Art
In semiconductor device fabrication, the various processing steps fall into four general categories: deposition, removal, patterning, and modification of electrical properties. Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Removal processes are any that remove material from the wafer either in bulk or selectively and consist primarily of etch processes, either wet etching or dry etching. For example, chemical-mechanical planarization (CMP) is a removal process that is used for planarizing a semiconductor wafer or other substrate. Patterning covers the series of processes that shape or alter the existing shape of the deposited materials and is generally referred to as lithography. For example, in photolithography, the wafer is cleaned and coated with a chemical called a “photoresist”. The photoresist is exposed by a “stepper”, a machine that focuses, aligns, and moves the mask, exposing select portions of the wafer to short wavelength light. The unexposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist may be removed by plasma ashing. Modification of electrical properties includes doping transistor sources and drains, annealing which serves to activate the implanted dopants and reducing the dielectric constant in low-k insulating materials.
One of the major obstacles in the semiconductor device fabrication process is the elimination of defects. For example, in chemical mechanical planarization it is advantageous to use smaller particles because smaller particles allow polishing to smoother surfaces, but the smaller particles tend to aggregate together. These aggregates often leave scratches, causing an uneven surface topology. Immersion lithography presents another example of the semiconductor fabrication process in which it is advantageous to eliminate defects. Early studies focused on the elimination of bubbles in the immersion fluid, temperature and pressure variations in the immersion fluid, and immersion fluid absorption by the photoresist (Switkes et al., 2003). In both examples, the method chosen to eliminate the defects focused on improving the chemical and physical properties of the fluid used in that process. The improvement of the chemical and the physical properties of the immersion fluid is particularly important in immersion lithography.
Immersion lithography is a lithography enhancement technique that replaces the usual air gap between the final lens element and the photoresist surface with a liquid medium with a refractive index (RI) greater than one. Integrated circuit manufacturers have accepted 193 nm immersion lithography (193i) as a manufacturing patterning solution at least down to the 45 nm half-pitch node. The advantage of immersion lithography can by understood using the relation between the resolution (R) of the technique, the wavelength (λ) of the light source used and the numerical aperture (NA) of the lens:R=k1λ/NA where k1 is a process dependent resolution factor. The numerical aperture can be expressed as n, sin θmax where n is the refractive index of the medium between the last lens element and the photoresist and θmax is the aperture angle. Thus, the resolution can be written as:R=k1λ/ni sin θmax which clearly shows that resolution is decreased when the refractive index of the immersion fluid is increased. As a result, a significant increase in the refractive index of the fluid could push immersion lithography down to the 32 nm half-pitch node. Water, with a refractive index nw=1.44 at 193 nm was initially used as a first generation immersion fluid. With water immersion, a maximum numerical aperture approaching nw=1.44 becomes possible. Water immersion scanners (193 nm) with numerical apertures of 1.3 or 1.35 are already commercially available. They can provide lithography solutions for line-and-space features as small as 45 nm half-pitch. But in order to achieve the 32 nm half-pitch node a 193 nm scanner with NA of at least 1.65 is needed (Sewell et al., 2006). The use of second generation fluids with a refractive index of approximately 1.65 leads to a numerical aperture around 1.5. However, to reach a numerical aperture close to 1.7, it is necessary to increase the refractive index of fluid.
Two different methods have been used to produce high-index fluids: a) addition of high refractive index additives in water and b) development of an organic, single component fluid (Wang et al., 2006). Organic fluids are usually based on cycloalkanes like decalin (French et al., 2006). However, there are issues with the chemical stability of these organic fluids, since they are susceptible to degradation upon exposure to atmosphere or during exposure to 193 nm radiation (Zimmerman et al., 2008). Other than chemical stability, the features that are necessary for the application of an immersion fluid are low absorption at 193 nm and viscosity that is not much higher than that of water. Aqueous based fluids containing nanoparticles are more likely to meet these requirements than organic based fluids (Chumanov et al., 2005; Rice, 2008). However, to realize these aqueous fluids, the nanoparticles should be smaller than 5 nm and should be made of a high refractive index material.
Additionally, the development of a high numerical aperture system requires a next generation photoresist that will be used in combination with the immersion fluid. The refractive index of this photoresist should be higher than 1.8. Initial approaches involved the insertion of highly polarizable atoms, like sulfur or phosphorus, on the organic compounds of the photoresist (Zimmerman et al., 2008). Also, using inorganic additives, like nanoparticles, in the immersion fluid, can be used to increase the refractive index of the photoresist.
Small and highly refractive index nanoparticles have chemical and physical properties that would eliminate the defects associated with slurries used in CMP. New polishing or cleaning processes are needed to achieve consistent nanoscale smoothness across larger diameter wafers in order to use these wafers to make, for example, 22 nm devices. Current 300 nm wafers are not smooth enough for these next-generation features. Additionally, 450 mm wafers may require making a surface with even less surface roughness across a 50% larger substrate, with more edge roll off, which will be markedly more difficult. Slurries comprising small nanoparticles will allow polishing to create smoother surfaces, but without defects since these smaller nanoparticles tend not aggregate.